1. Field of the Invention
The present invention relates to a cathode ray tube (CRT), and more particularly, to an electron gun for a CRT that can improve the efficiency of the magnetic field generated by a scanning velocity modulation (SVM) coil and effectively prevent the focus deterioration caused by an outer electric field.
2. Description of the Related Art
Generally, a CRT has a phosphor screen scanned by electron beams, a neck portion in which an electron gun for generating the electron beams is disposed, and a funnel portion for connecting the screen and the neck portion. Disposed around the neck portion is a deflection yoke for deflecting the electron beams generated by the electron gun. An SVM coil is also disposed around the neck portion to correspond to the electron gun.
The SVM coil is designed to improve the definition at borders of images by synchronizing the location of electron beams passing through electrodes of the electron gun with image signals. The SVM has two saddle coils facing each other and being interconnected in series. A brightness signal of the image signals is differentiated twice, and then input to the SVM coil.
A conventional electron gun used for a high definition projection CRT includes a cathode for radiating electrons, first to fifth grid electrodes for controlling the electrons radiated from the cathode, and a bead glass for supporting the grid electrodes. The grid electrodes are disposed inside the neck portion.
The first and second electrodes are formed as flat electrodes, and the third and fourth electrodes are formed as cylindrical electrodes. The fourth electrode is used as a focus electrode for focusing electron beams.
The SVM coil is disposed corresponding to the fourth electrode around the neck portion.
The cylindrical portion of the fourth electrode is located corresponding to the inner portion of the SVM coil, causing the magnetic field generated by the SVM coil to be blocked by the cylindrical portion of the fourth electrode and an eddy current to be generated on a metal surface of the cylindrical portion. This deteriorates the magnetic field efficiency affected on the electron beams, making it difficult to precisely control the location of the electron beams.
Since the location of the SVM coil is predetermined when designing the electron gun, it is difficult to displace the location of the SVM coil.
Accordingly, to improve the properties of the SVM coil, the number of coil turns should be increased or the amount of current should be increased. However, when increasing the number of coil turns, the size of the SVM is increased, and when increasing the current, the energy consumption is increased.
To solve the above-described problems, Japanese Laid-open Patent No. H8-115684 issued to Funakura for Electron Gun discloses an electron gun having two divided focus electrodes disposed having a gap (about 3 to 5 mm (millimeters)) there between so that the magnetic field generated in the SVM coil passes through the gap. As the magnetic field passes through the gap, the generation of the eddy current on the metal surface of the focus electrodes can be prevented, thereby improving the properties of the SVM coil.
However, an outer electric field (including an electric field generated by static electric fir charge accumulated on an inner wall of the neck portion) may be infiltrated through the gap, deteriorating the focusing operation of the focus electrode.
To solve this problem, the Japanese patent discloses, as another embodiment, an electron gun including two focus electrodes disposed facing each other with a gap between them. The electron gun further includes plural metal plates each having a thickness of about 0.2 to 0.5 mm (millimeter) attached to facing surfaces of the focus electrodes. The metal plates function as shield electrodes for preventing the eddy current by reducing the gap.
However, since plural metal plates are attached to each of the facing surfaces of the electrodes, the magnetic field generated by the SVM coil may be blocked by the plates, thereby generating the eddy current. In addition, since the gap between the metal plates cannot be defined having a sufficient distance, improvement of the properties of the SVM coil is limited.
To solve the above-described problems, Japanese Laid-open Patent No. H11-162372 to Nomura for Electron Gun discloses an electron gun having a focus electrode provided at its sidewall corresponding to the SVM coil with a slit perpendicular to the advancing direction of the electron beams so that the magnetic field generated by the SVM coil can pass through the slit.
The slit prevents the outer electric field from infiltrating as well as preventing the generation of the eddy current.
That is, since the magnetic field passes through the slit, the generation of the eddy current on the surface of the focus electrode is reduced, preventing the deterioration of the focusing property by the outer electric field. However, it is difficult to form the slit on the sidewall of the focus electrode, thereby increasing the manufacturing costs.